The invention relates to a boron carbide aluminum driveshaft tube. More specifically, it relates to driveshafts where the tubes are designed to achieve a desired level of physical properties for use in automotive applications.
Automotive driveshafts are carefully engineered to meet vehicle requirements. For example, driveshafts must withstand torque developed by the engine and transfer the torque to the drive wheels. Driveshafts must be sufficiently fatigue resistant to operate over the service life of the vehicle. Furthermore, the dimensions of the driveshafts, as well as their material compositions, are carefully designed in order to minimize vehicle weight and simplify vehicle design.
Metal matrix composites have become more practical and accepted for use in extruded driveshaft tubes. A commonly used material is aluminum metal matrix composite. This material includes particles of aluminum oxide in an aluminum alloy matrix. Such materials may be more difficult to weld, but are stronger and lighter than ordinary aluminum alloys.
U.S. Pat. No. 5,486,223 to Carden discloses a metal matrix composite that utilizes boron carbide as the ceramic additive to a base material metal. Carden discloses the use of the composite for the manufacture of structural members, with no suggestion to produce high performance extruded rotating members such as torque tubes for driveshafts.
Balancing of a driveshaft is a very important design requirement. In order to achieve a high degree of balance, the driveshaft tube is extruded with very close tolerances for straightness and circularity. After extrusion, the tube is typically taken up on rollers to achieve the final required dimensions. But even with careful workmanship, the center of mass of a rotating shaft never coincides perfectly with its axis of rotation. An imbalance results from this lack of coincidence. The imbalance generates a centrifugal force as the driveshaft rotates. The strength of the force increases as speed increases. At higher speeds, and thus at higher centrifugal forces, the imbalance can cause the shaft to deflect. In practice the effect of an imbalance can be minimized by adding balancing weights to the driveshaft assembly. Balancing of the driveshaft assembly also compensates for imbalances in the other driveshaft components.
Aside from the imbalance, every driveshaft has a critical speed at which the shaft exhibits resonance frequencies. That is, even when the imbalance problem is minimized by careful spin balancing of the driveshaft assembly, there is still a maximum speed that the driveshaft can rotate. At a critical speed, the shaft attempts to rotate both about its geometrical axis, and about the axis through its center of mass. Thus, driveshafts are designed to operate below this critical speed.
Automobiles consist of hundreds of component parts, all of which are packaged together in a vehicle to provide reliable and economical performance. Modern automotive design concepts place a premium on the efficient use of space, with an eye toward efficiency in manufacturing and an overall reduction in size and weight. The components must fit together within tight restraints relative to each other, with little flexibility for changing the dimensions of components once the overall design is set. The way the components fit together within tight restraints and close dimensions is referred to as packaging.
It is often necessary or desirable during the automotive design process to redesign parts at the last minute to accommodate material substitutions, increased demands for performance, enhanced availability of options for the consumer, or to take advantage of an increased performance available from new materials. It would be most desirable if such design changes could be accomplished with as little change to the packaging as possible.
Automotive driveshafts must often be designed and re-designed to accommodate changes in the design of the drive train. For example, if a new engine option is offered, the driveshaft must be changed so that the new torque requirements may be met. This can involve changing the outer and inner radii of the torque tube. However, changing the outer diameter of the tube leads to difficulties fitting the new re-designed tube within the packaging environment. Alternatively, it may be possible to reformulate the material of the tubes to accomplish a driveshaft with the same dimensions, but with different physical properties. However, this is not usually feasible because the reformulation would require a new ladle pour with its concomitant high expense and slow turn around time.
Thus, there is a need for a simplified way to deal with rapid changes in engineering requirements in the framework of packaging constraints in automobiles. The present invention provides such a solution.
In one embodiment, the invention provides a tube assembly containing an extruded torque tube and two end fittings such as yokes welded or otherwise attached to either end of the tube. The tube is made from a base material metal and boron carbide particles in a ratio of approximately between 2:1 to 10:1 by weight. In another embodiment, a driveshaft assembly includes the tube assembly above with a front universal joint and slip yoke attached to the front end, and a rear universal joint attached to the rear end. Additional objects and advantages of the invention will become apparent from the following detailed description of the preferred embodiment, the appended claims and accompanying drawings, or may be learned by practice of the invention.